The goal of these experiments is to determine the critical factors involved in the photoperiodic control of seasonal changes in body and lipid mass, metabolism, and energy expenditure in hamsters. The seasonal changes in body weight in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters are reflected entirely as changes in carcass lipid. These naturally- occurring changes in lipid mass are interesting in these animals because, although both species show the same seasonal reproductive responses, they show opposite changes in body weight and fat to the same environmental stimulus (daylength), mediated by the same neuroendocrine transducer (pineal gland and its hormone, melatonin). Three sets of experiments are proposed. The first set of experiments will describe the seasonal changes in lipid metabolism and their underlying mechanisms, and include measures of regional changes in white and brown adipose tissue lipogenesis and lipolysis, and the involvement of the sympathetic nervous system in these processes. The second set of experiments will examine the "regulation" of body weight, lipid mass and metabolism in Siberian hamsters, in particular the recovery of body and lipid mass, and adipose tissue cellularity changes following starvation-induced decreases in body weight, and the surgical removal of adipose tissue (lipectomy) performed during both the weight gain and loss phases of the body weight cycle. The third set of experiments will explore the possibility that melatonin directly affects hamster fat cell metabolism and that "secondary hormones" that are altered by the photoperiod (melatonin), in turn affect body and lipid mass changes due to changes in the daylength. Photoperiod induced changes in insulin, glucocorticoids and thyroid hormones will be documented, and experimentally manipulated to mimic the circulating levels associated with the photoperiod opposite to the photoperiod the animals are housed in. Changes in lipid mass and metabolism will be monitored. This multidisciplinary approach (utilizing concepts and techniques from endocrinology, biochemistry, neurosciences, zoology and psychobiology) should provide new information about the seasonal influences on body weight, fat, and lipid metabolism that may reflect fundamental processes in the development, maintenance and reversal of obesity.